Yes, reducing rake will also reduce trail, which is what you want.
Somewhere in the neighborhood of 4" trail is what you want.
He has almost double that!
With that being his first bike, I'd say put it back to stock!
I'm not for sure why you have a problem or why you call a 21" wheel a big wheel. after research and talking to a lot of people, I just changed from my original 18" front wheel to a 21". Only difference I feel is that it stands a little taller in the front (don't see as well over with wind screen at times). Now I will say that I haven't put it to much of a test. Only had it on for maybe 300 miles home. But like I said, I don't see much difference and all I've researched said you didn't have to make any adjustments going to a 21" wheel. Now 23" and above, modifications are necessary. And by the way, this new front wheel looks so much better than that stock wheel.